The two antitumor agents, Cyclophsophamide and Isophosphamide, are chiral organophosphorus compounds which require microsomal activation in vivo to their cytotoxic forms. The high selectivity of these cytotoxic metabolites to neoplastic tissue has been credited in part to the varying activity of additional metabolic processes which form non-toxic substances. Numerous stereochemical relationships could exist between the chiral enzymes involved and the chiral drugs or their metabolites which could result in a stereospecific partitioning among the metablic processes. We are investigating the importance of stereochemistry on the action of these clinically useful agents.